As the development of biometric devices for identity verification, and in particular of fingerprint sensing devices, has lead to devices which are made smaller, cheaper and more energy efficient, the possible applications for such devices are increasing.
In particular fingerprint sensing has been adopted more and more in, for example, consumer electronic devices, due to small form factor, relatively beneficial cost/performance factor and high user acceptance.
Capacitive fingerprint sensing devices, built based on CMOS technology for providing the fingerprint sensing elements and auxiliary logic circuitry, are increasingly popular as such sensing devices can be made both small and energy efficient while being able to identify a fingerprint with high accuracy. Thereby, capacitive fingerprint sensors are advantageously used for consumer electronics, such as portable computers, tablet computers and mobile phones, e.g. smartphones.
The fingerprint sensor may for example be arranged on the front surface of a handheld device, such as a smartphone or tablet computer, in which case the sensor is often arranged having the sensing surface in the same plane as a cover glass for the display of the device. To make the sensor blend in with, or to distinguish the fingerprint sensor from the cover glass, it may be desirable to select a particular color or pattern of the fingerprint sensor.
However, conventional color layers comprising a pigment are rather thick, sometimes more than 40 μm for achieving the desired color and glossiness. The thickness influences the sensor performance, since it increases the distance between the finger and the sensing chip. Currently known coating solutions using a pigment are also limited when it comes to the possibility of creating patterns comprising different colors since each color requires a separate processing step, and the resolution of the resulting pattern is limited to about 100 μm, depending on the application technology.
Furthermore, color pigments are molecules which only partially reflect the incident light and the color is defined by light absorption of molecules in the paint. Depending on the desired color, a molecule has to be found that has the required properties and fine tuning of colors is rather difficult. Moreover, as the color is depending on material properties, it may change as the pigment ages.